1. Field of the Invention
The present invention relates to a detector and method for detecting an abnormality in time synchronization, particularly for detecting a time lag between devices connected to each other through a communication line.
2. Description of the Related Art
A safety system for ensuring safety of a plant has been known. When the safety system detects an abnormality of the plant, the safety system generates an alarm and shuts down the plant to ensure the safety of the plant.
Further, in this safety system, an abnormality in a communication between safety controllers forming the system is constantly detected. When the communication abnormality is detected, the plant is also shut down. Thus, an unexpected state is avoided when the safety system does not function due to the communication abnormality.
JP-A-2000-305955 discloses an event collecting system for performing time correction so as to synchronize the time of a plurality of stations.
In the above-described safety system, an item for detecting the communication abnormality includes a transmission delay of the communication line. When a time from a transmission of a predetermined message to a reception of the message exceeds a predetermined threshold value, it is determined that there is some communication abnormality that causes the transmission delay, and the plant is shut down.
However, the transmitting time of the message is measured by a timer on a transmitting side and the receiving time of the message is measured by a timer on a receiving side, respectively. Therefore, time synchronization between the timers is not assured. When a time lag exists between the timers, there is a possibility that the communication abnormality due to the transmission delay is erroneously detected.